US Patent 7399523 Spinel ferrimagnetic particles and magnetic recording medium

Provided is a magnetic material with excellent magnetic properties to be used for recording media, which contains small ratio of superparamagnetic fine particles while maintaining high coercivity. It is spinel ferrimagnetic particles, a composition equation of which when prepared is (CoO)0.5-x(NiO)0.5-y(MO)x+y.n/2(Fe2O3) (M is a bivalent metal except Co and Ni) and a value of n (molar ratio)=Fe/(Co+Ni+Zn) is 2.0<n<3.0, which is larger than stoichiometric amount (n=2) of a spinel ferrite and less than that of 1.5 times, and values of x, y satisfy 0≦x<0.5, 0≦y<0.5, 0<x+y<0.5, wherein, also, superparamagnetic fine particles contained in the spinel ferrimagnetic particles is 5% by mass or less.